1. Field of the Invention
The present invention relates to a liquid chromatograph and a sample introducing apparatus therefor.
2. Description of the Related Art
A liquid chromatograph, which is shown in FIG. 1, is equipped with an analytical flow path consisting of an eluent holder 1, an eluent feeder 2, a separator 4, and a detector 5. The eluent feeder 2 feeds an eluent from the eluent feeder 2 to the separator 4.
The analytical flow path has a sample injector 3 placed between the eluent feeder 2 and the separator 4 (including a separation column). The sample injector 3 injects a sample into the flowing eluent, so that the sample is separated into components in the separator 4 and the separated components are detected by the detector 5.
The liquid chromatograph is usually constructed of several units, such as a pump (as the eluent feeder 2), an autosampler (as the sample injector 3), a separation column (as the separator 4), and a detector (as the detector 5).
The autosampler as the sample injector 3 is recently dominated by that of direct injection type. The autosampler of direct injection type causes the inside of the needle to constitute a part of the analytical flow path while the eluent is being fed under a high pressure for analysis.
The autosampler of direct injection type injects a sample in the following manner. First, the needle is detached from the analytical flow path under high pressure, and the detached needle functions as a part of the sample introduction flow path in the autosampler. The sample introduction flow path, which is connected to a syringe to suck up and discharge a sample in the autosampler, sucks up a sample through the needle as the plunger of the syringe is pulled out. After sample sucking, the sample introduction flow path, which holds the sample sucked up through the needle, is switched such that the analytical flow path for the eluent to flow from the pump communicates with the analytical flow path connected to the separation column and the sample held in the sample introduction flow path is injected into the analytical flow path. The autosampler of direct injection type employs a valve to switch the analytical flow path and the sample introduction flow path. It is common practice to place one valve each in the flow paths upstream and downstream the needle.
An ordinary autosampler of direct injection type performs sample injection in a manner which is explained below with reference to FIGS. 2 and 3.
In an ordinary autosampler of direct injection type, the flow path takes the route shown in FIG. 2 when a sample is discharged or sampling is not performed. The eluent delivered from the pump passes through the valve A 14 and then the needle 12. The eluent passes further through the injection port 13 and the valve A 14 again and flows toward the separation column.
Also, in an ordinary autosampler of direct injection type, the flow path takes the route shown in FIG. 3 when a sample is sucked up. At the time of sample suction, the valve A 14 acts to switch the flow path. The pump's action causes the eluent being delivered from the pump to flow directly to the separation column without passing through the needle 12 and the injection port 13. After separation from the flow path to the separation column, the needle 12 and the sample loop 22 are connected to the syringe 20 which moves the liquid in the autosampler. With the needle 12 inserted into the vial 23, the syringe 20 sucks up the sample so that the sample is held in the needle 12 and the sample loop 22.
After sample sucking, the needle 12 is connected to the injection port 13, and then the valve A 14 is switched again so that the flow path takes the previous route shown in FIG. 2. As the result, the eluent delivered from the pump enters the sample loop 22 and the needle 12, and the sample held in the needle 12 and the sample loop 22 is pushed into the injection port 13. Thus, the sample flows through the injection port 13 and the valve A 14 to reach the separation column.
As mentioned above, an autosampler of direct injection type is constructed such that both the upstream side and the downstream side of the needle are connected to the valve. This construction is disclosed in Patent Documents 1 and 2 below.